There is a small orifice in the arm over the primary bore. There will be a number stamped on the nozzle, probably 50 or 55, which equates to .50 or .55 mm. We are going to open up this up to .70 mm, to give a larger shot of fuel under acceleration. This will get rid of the stumble.
Take the same #70 drill and open up the existing orifice up to .70 mm. The hole is at a slight angle, and you will feel it break through into the supply passage that leads down the arm of the nozzle.
Blow out any chips and you are good to go. Re-install the nozzle, with a crush washer on both sides. Re-assemble the bowl cover and your Weber will perform quite admirably on your MGB.
